1. Field of the Invention
This invention relates to current amplifiers and specifically to operational transconductance amplifiers (OTA), and more specifically to chopper stabilized operational transconductance amplifiers.
2. Description of the Relevant Art
The problem addressed by this invention is encountered in designing a current amplifier with a low offset current. Conventional techniques for building low-offset OTA's through careful matching and layout have limited effectiveness in MOS circuits, where threshold variations can occur between otherwise matched devices. Threshold variations are independent of other device characteristics and therefore, can be nulled only by direct measurement of the offset and the subsequent cancellation of the offset.
FIG. 1 shows a conventional arrangement of two operational amplifiers (opamps) in which the offset of one opamp is measured and subsequently canceled using the feed forward technique known in the art as chopper stabilization and is described in the IEEE Journal of Solid State Circuits Vol. SC-16, No.6 December 1981. Using the circuit in FIG. 1, the nulling amplifier 10 alternately zeros itself and the main amplifier 12, storing correction voltages on capacitor 14 and 16. More specifically, capacitor 14 is charged with the error voltage of nulling amplifier 10 when switches 4 and 6 are closed and switches 2 and 8 are open. Then, switches 4 and 6 open while switches 2 and 8 close to charge capacitor 16 to the error voltage. These error voltages on capacitors 14 and 16 drive the offset adjustment inputs (n) of the amplifiers 10 and 12, respectively. When switches 4-6 and 2-8 are switched at high frequencies, the response of the system reduces to that of the main amplifier 12 alone.
While this the prior art technique is useful for voltage opamps, it is problematic when used on operational transconductance amplifiers. Additionally, it has been observed that switching switches as described in the prior art creates undesired noise in an OTA system.
Therefore, it is an object of the invention to have a current amplifier circuit with a low offset voltage.
It is further an object of this invention to have a current amplifier circuit with low noise.
It is further an object of the invention to improve upon prior art chopper stabilized circuits by modifying known techniques so that they can work with OTA's.